HEAD GASKET?

On the way to work my truck seemed to be slipping inot neutral. After finally getting to work I smelled burning and there was smoke under the hood. When I lifted the hood it was hissing and looked like chocolate milk had exploded under the hood and the same brownish fluid began puddling quicky under the right front headlight on the ground. Is this most likely my head gasket? If so, is this a pricey repair?

I would start with your cooling system with 205000 miles on the truck it probably needs to be flushed. And the thermastat replaced. You can take it and do a pressure check to see if there are any leaks. The brown fluid under the headlight is from the overflow tank. After you put more coolant back in how does the truck act? It could have a blown head gasket but I would start with the cooling system itaself especially the thermastat and flushing it out.